As integrated optics has developed, the use of various components peripherally related with such things as integrated lasers, switches, etc. has also developed. One of the most important peripheral elements is waveguide interconnects. Although optical fibers are utilized for long transmission lengths to interconnect various locations due to their minimum transmissive power loss, these would be classified as discrete elements and are impractical for an integrated circuit relating to optics.
While it has long been useful to fabricate integrated optic waveguides from other materials disposed on silicon, using its superior mechanical properties, it has become practical to fabricate the waveguides from silicon itself. In recent years, the efforts of the telecommunications industry to minimize dispersive power loss in fibers have driven carrier wavelengths further into the infrared spectrum. Presently, the commonly used wavelengths are 1.3 um and 1.55 um. Consequently, the need has arisen for integrated optical devices to operate at long wavelengths as well. Silicon itself is transparent in the 1.2-6.0 micrometer wavelength range and therefore provides some advantages as integrated waveguides in that it behaves as a low loss dielectric in its single crystalline, semi-insulating state, i.e., low-doped. Integrated waveguides are typically fabricated from dielectric slabs or channels that are clad or bounded by dielectrics with lower indices of refraction. This allows the light to propagate within the waveguide with very little attenuation due to the confinement of the light waves by total internal reflection. Silicon with its index of refraction of approximately 3.5 at a wavelength of 1.3 micrometer will form a waveguide when clad by silicon dioxide, which has an index of refraction of approximately 1.5.
One of the primary problems with disposing silicon dioxide about a silicon waveguide is the accessibility to the silicon surfaces during processing. In one process wherein the surfaces are accessible from top side processing, ridge structures are etched with or without subsequent back fill. This results in roughness due to etching process which leads to scattering losses when light reflects from the ridge walls. Further, this process utilizes air on the exterior with an index of refraction of 1.0 as compared to the index of refraction for silicon of 3.5, resulting in a large refractive index difference, and further enhancing scattering loss.
In another top side process, the ridge structures are etched and then a layer of thermal oxide formed on the etched surfaces. This provides some improvement in that the silicon dioxide has a higher refractive index as compared to air and the surface roughness is smoothed out by the oxidation reaction. Further, this silicon dioxide/silicon interface formed by the thermal oxidation tends to have silicon-rich transition layers near the boundary, resulting in a graded index profile.
Other processes for fabricating semiconductor waveguides are directed toward surfaces which are not accessible to top side processing, i.e., the undersides of the channels. In one process, a heavily doped substrate is utilized with the ridge waveguide formed on the upper surface thereof. This is illustrated in R. A. Soref et al., "Silicon Guided-Wave Optics", Solid State Technology, November, 1988, page 95. One problem with this type of structure is that the refractive index difference is very small such that a large fraction of transmitted power is actually carried in the heavily doped region if the waveguide above it is sufficiently thin. Heavily doped silicon, however, is a very absorptive material resulting in significant light attenuation.
Another structure for processing the underside of the waveguide utilizes the underlying layer of silicon dioxide with silicon deposited on the upper surface thereof. With this type of process, it is necessary to fabricate defect free monocrystalline silicon layers. Any degree of defectiveness or polycrystallinity will drastically increase absorption. This is typically referred to as silicon on insulator (SOI) technology.
Another type of SOI technology requires the formation of buried silicon dioxide by high energy implantation of oxygen ions through single crystal silicon, followed by high temperature annealing. This is referred to as the SIMOX process in Kurdi and Hall, "Optical Waveguides in Oxygen-Implanted Buried-Oxide Silicon-On-Insulator Structure", Optics Letters, February, 1988, volume 13, number 2, page 175. The problems with this type of system are the complexity and the size of the high energy implanters, residual damage in the silicon, and stress and deformation brought about by several factors, including volumetric expansion from the implanted oxygen, volumetric expansion from the subsequent silicon-to-silicon dioxide transition, and the extremely high temperature required for the transition.